Sea-Surface
Temperatures (SSTs) and
Mixed-Layer Conditions:
SSTs increased across most of the equatorial Pacific Ocean this
past month. The observed SST anomalies during May were close to
average in the western and central parts of the tropical Pacific
basin. In the mixed-layer, water temperatures were near-normal to
slightly above normal in the western and central Pacific, while
warm anomalies developed near the South American coast.

For the month, the SST anomaly in the Niño 3.4
Index region was +0.13°C (+0.23°F). The SSTs in the
Niño
4 Index region of the western equatorial Pacific were
+0.16°C (+0.29°F) above the mean (map of Niño
regions). For the most recent global ocean surface
temperatures, please see the loop of satellite-derived weekly SST anomalies
for May 2006.

Below average SST anomalies persisted throughout the latter half of
2005 and into the first few months of 2006 in the central
equatorial Pacific basin, and this can be seen in the data from
NCDC's Extended Reconstructed Sea Surface Temperature dataset
(ERSST version
2). Due to the recent warming of SSTs in the central tropical
Pacific, the 3-month running mean of the Niño 3.4 index
region increased above -0.5°C in April, and continued to warm
to near-average in May. (NOTE: For NOAA's official ENSO
classification scheme, please see NOAA's El
Niño/La Niña Index Definition and also see the
CPC ENSO Diagnostic Discussion for NOAA's latest official
assessment of ENSO conditions.)

Equatorial Zonal Winds (U-Component
Winds) and Sea-Level Topography:
The easterly trade winds were slightly below-normal across the
central and western equatorial Pacific basin during
May.
Stronger than average easterly winds were observed in the
eastern equatorial Pacific, where the trade winds were above
average during the month.

The most recent Pacific overpass of the NASA/JPL
Jason-1 satellite observed several areas with large positive
sea level anomalies in the southwest and northwest equatorial
Pacific, most likely associated with enhanced tropical convection
in these areas.

Sea levels were mostly near-average throughout the equatorial
Pacific during May, as the sea level heights reflected the
near-normal SST conditions at the end of the month (see the
30 May
2006 overpass).

Outgoing
Longwave Radiation (OLR):
The map to the left shows the spatial pattern of global OLR (in W
m-2) observed by satellite during May.
Positive OLR anomalies (typically associated with La Niña)
have persisted over the past several months primarily in the
equatorial western Pacific region, centered near the Date Line, and
tropical convection was again suppressed along the equator in the central
and eastern Pacific this past month. The 3-month
averaged OLR anomalies were also positive in the western
Pacific basin and along the equator, reflecting the reduced
cloudiness during the 2005/2006 cold event.

The monthly-averaged OLR Index was positive during May, but
remained only slightly above neutral with a value of +0.9 W
m-2 for the month, averaged across an area centered over
the Date Line in the western Pacific between 160° E and
160° W (note that positive OLR Index values are typically
associated with La Niña conditions in this region). May was
the tenth consecutive month with a positive OLR Index.

Note that high frequency variability in OLR is typically associated
with the Madden-Julian
Oscillation (MJO, which is convective activity that propagates
west to east in the near-equatorial region from the Indian Ocean
into the Pacific Ocean approximately every 30-60 days). The latest
MJO activity can be seen in CPC's graphs of
Daily MJO Indices.

Southern Oscillation Index
(SOI):
The standardized SOI switched signs to a negative value in May, with an
average value of -0.8 for the month. The SOI was inconsistent
during the relatively weak 2005/2006 La Niña, fluctuating
several times between a near-neutral value and both positive and
negative indices. The recent shift to a negative value in May
coincided with the development of warmer than average SSTs in the
equatorial Pacific.

Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: El Niño/Southern Oscillation for May 2006, published online June 2006, retrieved on August 2, 2015 from http://www.ncdc.noaa.gov/sotc/enso/200605.